1. Opponent-color ganglion cells of macaques can be classed as X or Y. Cells with a cone-specific receptive-field organization (center and surround receiving input from spectrally different cone types) have a linear summation, whereas cells with a cone-mixed organization (center and surround partly mediated by input from the same cone type) have a nonlinear summation. 2. Pure center and pure surround responses of Y-cells have a fast decay and show conspicuous transients at stimulus offset and onset; pure responses of X-cells have a slow decay and show fewer transients, especially at stimulus offset. 3. Sensitivity profiles based on pure responses elicited in conditions of chromatic adaptation of the opponent responses show that Y-cells have unimodal center and unimodal surround profiles, whereas X-cells have unimodal center and bimodal surround profiles. 4. Responses receiving contribution from both opponent mechanisms (mixed) have different time course and pattern in X- and Y-cells. Mixed response of Y-cells show a discontinuity in cell firing during the transient (on) component of cell activity, which has a higher sensitivity than other waveform changes produced by concurrent stimulation of the opponent mechanism. This discontinuity occurs with stimulus conditions that also elicit proximal negative responses in the local electroretinogram and appears to be due to a centrally located process having some degree of rectification.